Microfluidic particle separation is widely used in sorting, purification, enrichment and detection of cells in cell biology, drug discovery, and clinical diagnostics. A number of methods currently exist for particle separation on microfluidic platforms, such as size-based separation, acoustic separation, dielectrophoresis, fluorescence-activated and magnetic-activated separation. Among these, methods based on magnetic control are particularly attractive. These methods utilize surface-functionalized magnetic beads to capture target microparticles and to separate the target microparticles by magnetic manipulation. This separation scheme can be based on specific binding between the magnetic beads and the target microparticles instead of relying on geometrical or physical properties of the particles, and hence allows highly specific and selective particle separation.
There are two well known operating modes for magnetically based microfluidic particle separation: batch mode and continuous flow mode. In the batch mode, target-bound magnetic beads are retained on a solid surface, and subsequently released following the removal of non-target microparticles with a liquid phase. Magnetic bead beds and sifts have for example been developed for this purpose, but have limited separation efficiency. A number of devices have been attempted to address this issue with various magnet designs, including a quadruple electromagnet, a planar electromagnet, or nickel posts. In addition, planar electromagnets can be integrated on chip with microvalves and micropumps to enable fully automated functionalities such as fluid actuation and particle mixing. Many batch-mode designs suffer from several inherent limitations including prolonged durations of operation, complicated fluidic handling, and contamination due to non-specific trapping of impurities that are sequestered in the beads.
These limitations can be mitigated by continuous-flow magnetic bead separation, which employs magnetic fractionation, i.e., continuous accumulative deflection of magnetic beads. This method can be classified into two categories depending on whether an integrated magnet or off-chip magnet is used. In the first category, magnetic microstrips of alloy or ferromagnetic materials are deposited on the device substrate for generating a magnetic field gradient to separate magnetic beads. However, this on-chip integrated magnet microstrip design typically requires sophisticated design and fabrication to achieve a proper balance between hydrodynamic and magnetic forces. Alternatively, the use of a simple external magnetic setup in conjunction with on-chip separation allows greater flexibility in device design and magnetic manipulation. In either case, however, magnetic separators can be limited by prolonged off-chip incubation of target microparticles with magnetic beads, which is used to ensure sufficient bead-particle interaction and binding before on-chip separation. This off-chip incubation is time-consuming, labor-intensive, and prone to contamination, and in the case of cell separation could compromise the viability of target cells.
There exists a need to provide simple yet effective on-chip magnetically based systems and methods for capturing and separating target microparticles.